The present invention relates to a novel heat resisting steel and, more particularly, to a heat resisting steel suitable for use as the material of turbine casing, main steam stop valve and steam regulating valve of steam turbine for large-scale thermal power generation.
Nowadays, the steam turbines for thermal power generation are required to operate at the maximum steam temperature and pressure of 538.degree. C. and 246 atm. In order to withstand such a severe steam condition, the casing, etc. of the steam turbine are made of a Cr-Mo-V cast steel.
On the other hand, recently the improvement in the power generating efficiency in the power plant is becoming more significant, in view of the shortage of fossil fuels such as petroleum and coal, as well as the trend for saving of natural resources. Effective measures for attaining higher power generating efficiency are to raise the steam temperature or pressure and to increase the size of the steam turbine. When these measures are adopted, however, the conventional method used for turbine casing, etc., i.e. the Cr-Mo-V cast steel mentioned above, cannot provide a sufficient high-temperature strength. This gives rise to the demand for material having a greater high-temperature strength.
The present inventors made a study on a steel basically composed of Cr-Mo-V steel with addition of a very small amount of B, as a material which can safely be used when the steam turbine is large and the temperature and pressure of steam are increased. Although the addition of a very small amount of B improves the hardenability of the steel and provides a remarkable improvement in the high-temperature strength, it impairs the weldability and, in particular, undesirably increases the cracking sensitivity (SR cracking sensitivity), i.e. the sensitivity to cracking which occurs in a heat affected zone when the weld zone is subjected to a stress relief annealing after the welding. The material for the casing, steam regulating valve and main steam stop valve of steam turbine for thermal power generation has to have a high resistance to SR cracking, because these parts are integrated by welding and subjected to stress relief annealing after the welding.
Japanese Patent Application Laid-Open Publication No. 41962/80 discloses a Cr-Mo-B steel. This steel, however, does not contain V and, therefore, is low in high-temperature strength, particularly in creep rupture strength, and cannot be suitable for steam of high temperature of 593.degree. C. In addition, in the above Publication nothing is disclosed at all about the weldability.
The specification of U.S. Pat. No. 3,316,084 discloses a Cr-Mo-V steel containing Al. This steel, however, does not contain B and, therefore, cannot provide a sufficiently high creep rupture strength.